Adds a new package that rewrites the schema-level constructs whose
representation changed between OpenAPI 3.0 and 3.1, in place. The
transformation is mechanical and lossless — every 3.0 construct has a
direct 3.1 form. Use this when a downstream consumer (diff tools,
validators, code generators) needs a single canonical representation
regardless of the source spec's declared version.

Transformations (per the OAI upgrade guide):

  openapi: 3.0.x                          -> openapi: 3.1.1 (configurable)
  nullable: true + type: T                -> type: [T, "null"]
  nullable: true + type: [T, U]           -> type: [T, U, "null"] (deduped)
  nullable: true (no type)                -> drop nullable
  minimum: x + exclusiveMinimum: true     -> exclusiveMinimum: x; minimum cleared
  maximum: x + exclusiveMaximum: true     -> exclusiveMaximum: x; maximum cleared
  exclusiveMinimum: false / exclusiveMaximum: false  -> dropped (default)
  Schema example: v                       -> examples: [..., v]

API:

  func UpgradeTo31(doc *openapi3.T) error
  func UpgradeTo31WithOptions(doc *openapi3.T, opts UpgradeOptions) error
  func UpgradeSchema(s *openapi3.Schema)        // sub-tree variant

  type UpgradeOptions struct {
      SkipVersionBump bool      // leave doc.OpenAPI alone
      TargetVersion   string    // defaults to "3.1.1"
      Verbose         io.Writer // log each rewrite
  }

In-place mutation matches the use case (kin-openapi shares one
openapi3.T type between 3.0 and 3.1; returning a deep-copied tree
would be wasteful and error-prone for $refs and components). The walk
is cycle-safe via a visited-pointer set.

Tests cover all transformations, idempotence on already-3.1 docs,
operation-tree walking, cycle safety, verbose logging, and nil safety.

Out of scope for this PR (lower priority, can land later):
- File-upload normalization (format: byte / format: binary)
- $schema dialect declaration on Schema Objects
- Direct 2.0 -> 3.1 (compose openapi2conv.ToV3 then this package)

Five of the six items from getkin#1162's review:

- logf: use Fprintln(w) for the trailing newline instead of editing the
  format string (line 135).
- walkDoc: drop the superfluous nil-before-Map() check (line 184) — the
  Map() helper already handles nil Paths internally.
- rewriteNullable: replace the hand-rolled "already has null" loop with
  slices.Contains (line 316).
- rewriteNullable: simplify the type-array append (line 320). The earlier
  copy-then-append guarded against caller-side aliasing of *s.Type, but
  s.Type is reassigned immediately after, so the simpler in-place append
  is fine.
- Drop SkipVersionBump from UpgradeOptions. Callers that want the
  representations canonicalized while keeping the original version string
  can do `original := doc.OpenAPI; openapi3conv.Upgrade(...); doc.OpenAPI = original`
  themselves — cheap escape hatch, doesn't justify a public option.
  Removes the corresponding test.

The sixth item (drop Target / drop error return / private options) is
held pending the design discussion on PR thread.

Per the design discussion on the PR:

- Drop the Target option. The OAI commits to strict compatibility for
  3.x going forward (3.2.1 and 3.3.0 milestones), so a tool that handles
  3.1+ correctly handles all later 3.x versions correctly too. The 3.0 →
  3.1 transition is the only break and is exactly what this package
  bridges. Always upgrade to the latest 3.x version the package knows
  about (3.2.0 today; bumped via the internal latestTargetVersion const
  when a new minor lands).
- Drop the error return. Documents must be Validate()'d before calling
  Upgrade; with that invariant, the only failure modes the function had
  (cross-major upgrade, downgrade, malformed Target) are all eliminated.
- Make options private. UpgradeOptions becomes the unexported
  upgradeOptions, accessed via functional Option-style accessors.
  WithWriter is the only initial option; future options are added by
  introducing a new WithX function rather than expanding a public
  struct.
- Drop UpgradeTo31 (no longer adds anything over Upgrade) and
  DefaultTargetVersion (latestTargetVersion is the internal replacement).
- Drop parseVersion (only used by the cross-major / downgrade checks
  that are gone) and the strconv/strings imports.
- Update package docstring to reflect the new always-latest model and
  the OAI compatibility commitment.

Tests rewritten to match the new API: the Target / error-path tests
are removed (no longer reachable), TestUpgradeTo31_* renamed to
TestUpgrade_*, the cycle test uses time.After for an actual timeout,
and the verbose test uses WithWriter.

Closes the design thread on PR getkin#1162.

Per review feedback: most Upgrade tests now use upgradeAndAssertValid
which calls .Validate() on both sides — input must validate as 3.0,
output must validate as latest 3.x. Two tests opt out (and document
why) because they intentionally feed a 3.0-stamped doc that contains
3.1-only constructs to exercise the rewrite logic.